The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years.

In low temperature plasmas, the interaction of the electrons with the electric field is an important current research topic that is relevant for many applications. Particularly, in the low pressure regime (≤10 Pa), electrons can traverse a distance that may be comparable to the reactor dimensions without any collisions. This causes “nonlocal,” dynamics which results in a complicated space- and time-dependence and a strong anisotropy of the distribution function. Capacitively coupled radio frequency (CCRF) discharges, which operate in this regime, exhibit extremely complex electron dynamics. This is because the electrons interact with the space- and time-dependent electric field, which arises in the plasma boundary sheaths and oscillates at the applied radio frequency. In this tutorial paper, the fundamental physics of electron dynamics in a low pressure electropositive argon discharge is investigated by means of particle-in-cell/Monte Carlo collisions simulations. The interplay between the fundamental plasma parameters (densities, fields, currents, and temperatures) is explained by analysis (aided by animations) with respect to the spatial and temporal dynamics. Finally, the rendered picture provides an overview of how electrons gain and lose their energy in CCRF discharges.

The effect of changing the driving frequency on the plasma density and the electron dynamics in a capacitive radio-frequency argon plasma operated at low pressures of a few Pa is investigated by particle-in-cell/Monte-Carlo collision simulations and analytical modeling. In contrast to previous assumptions, the plasma density does not follow a quadratic dependence on the driving frequency in this non-local collisionless regime. Instead, a step-like increase at a distinct driving frequency is observed. Based on an analytical power balance model, in combination with a detailed analysis of the electron kinetics, the density jump is found to be caused by an electron heating mode transition from the classical α-mode into a low-density resonant heating mode characterized by the generation of two energetic electron beams at each electrode per sheath expansion phase. These electron beams propagate through the bulk without collisions and interact with the opposing sheath. In the low-density mode, the second beam is found to hit the opposing sheath during its collapse. Consequently, a large number of energetic electrons is lost at the electrodes resulting in a poor confinement of beam electrons in contrast to the classical α-mode observed at higher driving frequencies. Based on the analytical model this modulated confinement quality and the related modulation of the energy lost per electron lost at the electrodes is demonstrated to cause the step-like change of the plasma density. The effects of a variation of the electrode gap, the neutral gas pressure, the electron sticking and secondary electron emission coefficients of the electrodes on this step-like increase of the plasma density are analyzed based on the simulation results.

The kinetic origin of resonance phenomena in capacitively coupled radio frequency plasmas is discovered based on particle-based numerical simulations. The analysis of the spatio-temporal distributions of plasma parameters such as the densities of hot and cold electrons, as well as the conduction and displacement currents reveals the mechanism of the formation of multiple electron beams during sheath expansion. The interplay between highly energetic beam electrons and low energetic bulk electrons is identified as the physical origin of the excitation of harmonics in the current.Capacitively coupled radio frequency (CCRF) discharges are indispensable tools for semiconductor manufacturing and other innovative applications [1,2]. At the same time, they are challenging physical systems due to their complex and nonlinear dynamics. At low neutral gas pressures of a few Pa or less, CCRF discharges are operated in a strongly non-local regime. In the so-called "α-mode", electron heating is dominated by stochastic sheath expansion heating [3] and electric field reversal during sheath collapse [4][5][6][7]. Stochastic heating was modelled extensively in the past in the frame of a hard wall model, as well as pressure heating [8][9][10][11][12][13][14][15][16][17]. During the phase of sheath expansion, energetic electron beams are generated and propagate into the plasma bulk, where they sustain the discharge via ionization and lead to a Bi-Maxwellian electron energy distribution function (EEDF) [18][19][20][21][22][23][24]. At low pressures, resonance effects such as the plasma series resonance (PSR) [27][28][29][30] and the plasma parallel resonance (PPR) [31][32][33] can be self-excited and strongly enhance the electron heating [19,29]. In the presence of a sinusoidal driving voltage waveform, the excitation of the PSR results in a non-sinusoidal RF current, due to the appearance of harmonics of the driving frequency [30]. Although these have been observed in experiments [21], they are usually neglected in most models of electron heating in CCRF plasmas. Existing theories which include resonance effects are zero-dimensional global models based on equivalent electrical circuits [29] as well as spatially resolved models based on the cold plasma approximation [30]. As these models do not include any kinetic effects, such resonances should be investigated on a microscopic kinetic level. A kinetic interpretation is required to clarify some of the most important open questions about electron heating dynamics in CCRF plasmas: What is the kinetic origin of the generation of high frequency (HF) oscillations of the RF current and the generation of multiple electron beams during one phase of sheath expansion such as observed in previous works [37]? In what way is current continuity (∇ · j tot = 0) ensured at all times within the RF period in the presence of electron beams, where the total current density j tot = j d + j c is decomposed into the displacement and conduction current density? Our aim is to provide access to a kinetic inter...